EQMResearch group
Level 2 · How matter responds to light

Semiconductors & band gap

In crystals, electron levels merge into bands. The gap between the full and empty bands sets the lowest photon energy that can be absorbed.

Build on:Atoms & electrons

From rungs to ribbons

Pile lots of atoms into a crystal and the discrete energy rungs of each atom merge into wide bands. The highest filled band is called the valence band; the next-empty one above it is the conduction band; the gap between them — the band gap Eg — is what makes the crystal a semiconductor.

Valence band (full)Conduction band (empty)Eg = 1.50 eVhν = 1.50 eVe⁻h⁺

Below the gap, the crystal is transparent: there is no available electronic state for the photon to drop into. Above the gap, an electron is kicked from the valence band into the conduction band, leaving behind a hole.

Below the gap (yellow photon) the crystal is transparent. Above the gap, an electron is pushed into the conduction band, leaving a hole behind.

Why we care

Three direct consequences for this experiment:

  • CrSBr's band gap (~1.5 eV) sets the energy below which the material itself is roughly transparent.
  • Just below the gap an exciton appears — a bound state that gives the sharpest absorption feature in the system.
  • The whole optical fingerprint of CrSBr (its dielectric function) is dominated by physics happening near this gap.
Key takeaways
  • Crystals quantise allowed energies into bands.
  • The band gap controls absorption onset and exciton position.
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